1. Field of the Invention
Generally, the present disclosure relates to the fabrication of highly sophisticated integrated circuits including advanced transistor elements that comprise highly capacitive gate structures including a high-k gate dielectric of increased permittivity compared to gate dielectrics, such as silicon dioxide and silicon nitride.
2. Description of the Related Art
The fabrication of advanced integrated circuits, such as CPUs, storage devices, ASICs (application specific integrated circuits) and the like, requires the formation of a large number of circuit elements on a given chip area according to a specified circuit layout, wherein field effect transistors represent one important type of circuit elements that substantially determine performance of the integrated circuits. Generally, a plurality of process technologies are currently practiced, wherein, for many types of complex circuitry, including field effect transistors, MOS technology is currently one of the most promising approaches due to the superior characteristics in view of operating speed and/or power consumption and/or cost efficiency. During the fabrication of complex integrated circuits using, for instance, MOS technology, millions of transistors, e.g., N-channel transistors and/or P-channel transistors, are formed on a substrate including a crystalline semiconductor layer. A field effect transistor, irrespective of whether an N-channel transistor or a P-channel transistor is considered, typically comprises so-called PN junctions that are formed by an interface of highly doped regions, referred to as drain and source regions, with a slightly doped or non-doped region, such as a channel region, disposed adjacent to the highly doped regions.
In a field effect transistor, the conductivity of the channel region, i.e., the drive current capability of the conductive channel, is controlled by a gate electrode formed adjacent to the channel region and separated therefrom by a thin insulating layer. The conductivity of the channel region, upon formation of a conductive channel due to the application of an appropriate control voltage to the gate electrode, depends on the dopant concentration, the mobility of the charge carriers and, for a given extension of the channel region in the transistor width direction, on the distance between the source and drain regions, which is also referred to as channel length. Hence, in combination with the capability of rapidly creating a conductive channel below the insulating layer upon application of the control voltage to the gate electrode, the conductivity of the channel region substantially affects the performance of MOS transistors. Thus, as the speed of creating the channel, which depends on the conductivity of the gate electrode, and the channel resistivity substantially determine the transistor characteristics, the scaling of the channel length, and associated therewith the reduction of channel resistivity and increase of gate resistivity, is a dominant design criterion for accomplishing an increase in the operating speed of the integrated circuits.
Presently, the vast majority of integrated circuits are based on silicon due to substantially unlimited availability, the well-understood characteristics of silicon and related materials and processes and the experience gathered during the last 50 years. Therefore, silicon will likely remain the material of choice for future circuit generations designed for mass products. One reason for the dominant importance of silicon in fabricating semiconductor devices has been the superior characteristics of a silicon/silicon dioxide interface that allows reliable electrical insulation of different regions from each other. The silicon/silicon dioxide interface is stable at high temperatures and, thus, allows the performance of subsequent high temperature processes, as are required, for example, for anneal cycles to activate dopants and to cure crystal damage without sacrificing the electrical characteristics of the interface.
For the reasons pointed out above, silicon dioxide is preferably used as a gate insulation layer in field effect transistors that separates the gate electrode, frequently comprised of polysilicon or other metal-containing materials, from the silicon channel region. In steadily improving device performance of field effect transistors, the length of the channel region has continuously been decreased to improve switching speed and drive current capability. Since the transistor performance is controlled by the voltage supplied to the gate electrode to invert the surface of the channel region to a sufficiently high charge density for providing the desired drive current for a given supply voltage, a certain degree of capacitive coupling, provided by the capacitor formed by the gate electrode, the channel region and the silicon dioxide disposed therebetween, has to be maintained. It turns out that decreasing the channel length requires an increased capacitive coupling to avoid the so-called short channel behavior during transistor operation. The short channel behavior may lead to an increased leakage current and to a dependence of the threshold voltage on the channel length. Aggressively scaled transistor devices with a relatively low supply voltage and thus reduced threshold voltage may suffer from an exponential increase of the leakage current, while also requiring enhanced capacitive coupling of the gate electrode to the channel region. Thus, the thickness of the silicon dioxide layer has to be correspondingly decreased to provide the required capacitance between the gate and the channel region. For example, a channel length of approximately 0.08 μm may require a gate dielectric made of silicon dioxide as thin as approximately 1.2 nm. Although, generally, high speed transistor elements having an extremely short channel may preferably be used for high speed applications, whereas transistor elements with a longer channel may be used for less critical applications, such as storage transistor elements, the relatively high leakage current caused by direct tunneling of charge carriers through an ultra-thin silicon dioxide gate insulation layer may reach values for an oxide thickness in the range or 1-2 nm that may not be compatible with requirements for performance-driven circuits.
Therefore, replacing silicon dioxide as the material for gate insulation layers has been considered, particularly for extremely thin silicon dioxide gate layers. Possible alternative materials include materials that exhibit a significantly higher permittivity so that a physically greater thickness of a correspondingly formed gate insulation layer provides a capacitive coupling that would be obtained by an extremely thin silicon dioxide layer. Commonly, a thickness required for achieving a specified capacitive coupling with silicon dioxide is referred to as capacitance equivalent thickness (CET). Thus, at a first glance, it appears that simply replacing the silicon dioxide with high-k materials is a straightforward way to obtain a capacitance equivalent thickness in the range of 1 nm and less.
It has thus been suggested to replace silicon dioxide with high permittivity materials such as tantalum oxide (Ta2O5) with a k of approximately 25, strontium titanium oxide (SrTiO3) having a k of approximately 150, hafnium oxide (HfO2), HfSiO, zirconium oxide (ZrO2) and the like.
Additionally, transistor performance may be increased by providing an appropriate conductive material for the gate electrode to replace the usually used polysilicon material, since polysilicon may suffer from charge carrier depletion at the vicinity of the interface to the gate dielectric, thereby reducing the effective capacitance between the channel region and the gate electrode. Thus, a gate stack has been suggested in which a high-k dielectric material provides enhanced capacitance based on the same thickness as a silicon dioxide layer, while additionally maintaining leakage currents at an acceptable level. On the other hand, the non-polysilicon material, such as titanium nitride and the like, may be formed so as to connect to the high-k dielectric material, thereby substantially avoiding the presence of a depletion zone. Since, typically, a low threshold voltage of the transistor, which represents the voltage at which a conductive channel forms in the channel region, is desired to obtain the high drive currents, the controllability of the respective channel requires pronounced lateral dopant profiles and dopant gradients, at least in the vicinity of the PN junctions. Therefore, so-called halo regions are usually formed by ion implantation in order to introduce a dopant species whose conductivity type corresponds to the conductivity type of the remaining channel and semiconductor region so as to “reinforce” the resulting PN junction dopant gradient after the formation of respective extension and deep drain and source regions. In this way, the threshold voltage of the transistor significantly determines the controllability of the channel, wherein a significant variance of the threshold voltage may be observed for reduced gate lengths. Hence, by providing an appropriate halo implantation region, the controllability of the channel may be enhanced, thereby also reducing the variance of the threshold voltage, which is also referred to as threshold roll off, and also reducing significant variations of transistor performance with a variation in gate length. Since the threshold voltage of the transistors is significantly determined by the work function of the metal-containing gate material, an appropriate adjustment of the effective work function with respect to the conductivity type of the transistor under consideration has to be guaranteed.
After forming sophisticated gate structures including a high-k dielectric and a metal-based gate material, however, high temperature treatments may be required, which may result in a shift of the work function and a reduction of the permittivity of the gate dielectric, which may also be associated with an increase of layer thickness, thereby offsetting many of the advantages of the high-k dielectric in combination of the metal material. It is believed that the deterioration of the high-k metal gate is substantially caused by the incorporation of oxygen and a respective oxygen diffusion within the high-k dielectric material, wherein the oxygen diffusion may be fed by oxygen contained in the ambient and in any neighboring materials, such as silicon dioxide and the like, that may come into contact with the high-k dielectric during the processing of the devices. Since, for instance, hafnium- and zirconium-based oxides grow very fast due to the high affinity to oxygen even at moderately high temperatures, a significant modification of the characteristics of the high-k dielectric material may be observed, for instance, an increased layer thickness and thus a reduced dielectric constant, which may be even further pronounced at moderately high temperatures of approximately 950-1300° C., as may typically be used during activation treatments and the like.
In addition to a significant modification of the high-k dielectric material, the work function of the metal in the gate stack may also be shifted towards the center of the band gap, thereby modifying the threshold voltage of respective transistors. Due to the high oxygen affinity of the high-k dielectric material, the gate stack is usually encapsulated after the patterning process in order to avoid, or at least significantly reduce, any contact of oxygen contained in the process ambience and in neighboring material, such as silicon dioxide and the like, so as to enhance stability of the high-k dielectric material and the respective metals in the gate stack. For this purpose, silicon nitride has proven to be a promising material due to its oxygen-blocking characteristics. Hence, in typical conventional process flows, a silicon nitride liner with a thickness in the range of approximately 1-5 nm may be formed on exposed surface areas of the patterned high-k gate stack, wherein appropriate deposition techniques are used so as to not unduly affect device characteristics and/or the subsequent manufacturing steps. That is, well-established low pressure chemical vapor deposition (LPCVD) techniques for silicon nitride may require temperatures of approximately 750° C. and above in order to obtain the desired thermally activated deposition mechanism. However, at this moderately high process temperature, to which the sensitive gate stacks may have to be exposed prior to the actual deposition process, a significant degree of oxidation may occur, due to remaining oxygen residuals, and an unwanted ammonia nitridation of the exposed surfaces may also take place, which may result in highly non-uniform material characteristics of the exposed surface portions. For this reason, typical process temperatures at approximately 500° C. and less may be used for forming the silicon nitride material, which may be accomplished on the basis of sophisticated chemical vapor deposition (CVD) techniques, such as atomic layer deposition (ALD) or other cyclic deposition techniques, in which the precursor materials may be provided as a sequence of layers, wherein both precursor layers may provide a substantially self-limiting deposition behavior, as is the case in ALD strategies, thereby providing a high degree of controllability and conformality, however, at the cost of moderately long process times. Similarly, in other techniques, one of the precursor layers may not provide a self-limiting deposition behavior, yet nevertheless provide enhanced conformality and controllability due to the moderately long process times. A respective deposition technique, in which two or more precursor layers may be sequentially deposited, however, without requiring a self-limiting behavior, may be referred to herein as “multilayer deposition.” Thus, these deposition techniques may allow the deposition of a conformal and thin silicon nitride layer with a high degree of controllability. Furthermore, plasma enhanced chemical vapor deposition (PECVD) at temperatures of approximately 400-500° C. may also be used, thereby obtaining high deposition rates at low process temperatures, wherein, however, control of layer thickness may be difficult in this range of thickness, thereby rendering the PECVD technique less attractive for encapsulating high-k gate stacks. Although the sophisticated multi-layer deposition approach and the ALD technique may provide superior controllability of layer thickness, the etch selectivity of the respective silicon nitride materials with respect to wet chemical etch chemistries for etching silicon dioxide, which is frequently used during the semiconductor manufacturing flow, may be moderately low due to the moderately low deposition temperature. Similarly, PECVD silicon nitride may also exhibit a reduced etch selectivity with respect to the wet chemical etching of silicon dioxide material.
The present disclosure is directed to various methods and devices that may avoid, or at least reduce, the effects of one or more of the problems identified above.